This invention relates to an alarm system utilizing a bidirectional wired television system.
According to already proposed alarm system utilizing a CATV system, the alarm signals from respective subscriber's ends are sent out from an alarm transmitter as low frequency signals having different frequencies for respective subscribers and the low frequency signals are grouped for suitable blocks and are modulated with high frequencies which are different for respective blocks to form incoming signals which are transmitted to an alarm receiving panel. The alarm receiving panel is provided with a circuit for discriminating the subscriber's ends on the transmission side. This discriminating circuit comprises a demodulator including a high frequency band pass filter and a plurality of low frequency band-pass filters. The demodulator which is provided with the high frequency bandpass filter functions to demodulate modulated signals which have been modulated with high frequencies different for respective groups as described above. Accordingly, it is necessary to use a plurality of demodulators of the same number as that of the modulators. Signals grouped into a plurality of blocks by the demodulators provided with bandpass high frequency filters are finally separated by a plurality of low frequency bandpass filters to operate respective display devices respectively corresponding to the original subscriber's ends.
This system, however, requires the use of a plurality of low frequency bandpass filters for each demodulator. Further, it is also necessary to install a plurality of low frequency bandpass filters of the same number as that of the subscriber's ends for the receiving panel. Accordingly, as the number of subscriber's ends increases the cost of the alarm system also increases and accurate separation of the signals becomes difficult, thus causing faulty operation as well as erroneous display.
U.S. Pat. No. 3,996,578 discloses an alarm system constructed to overcome the above-mentioned deficiencies. In the alarm system of this patent, the alarm receiving panel is provided with a plurality of demodulators for respective groups, a plurality of first bandpass filters corresponding to the high frequency carrier signals utilized for modulation and connected to apply modulated alarm signals to the demodulators, a plurality of first display means responsive to the output of the demodulators to provide first displays for the respective groups, a plurality of second bandpass filters corresponding to the low frequencies for the respective subscribers, means for supplying the outputs of all of the demodulators in common to all of the second bandpass filters, and a plurality of second display means connected to the outputs of the second bandpass filters to provide second displays for the respective subscribers. With this alarm system, the subscriber which has transmitted an alarm signals is indicated by the combination of the first and second displays, that is, the first display discriminates the corresponding group and the second display discriminates the corresponding subscriber in the group. A disadvantage of this alarm system is that while it is possible to discriminate one or plurality of subscribers in one group, if this group is the one and only group indicated by the first display, if two or more groups are displayed by the first displays and at the same time two or more subscribers are displayed by the second displays, the system is not able to discriminate the relationship between the groups and the subscribers.